FR2982618A1 - METHOD FOR MANUFACTURING TITANIUM ALLOY FOR BIOMEDICAL DEVICES - Google Patents

Abstract

A process for producing a titanium alloy with super elastic properties and / or shape memory for biomedical application, comprising the phases of: - preparation of an ingot by vacuum melting the various constituent metals of the desired alloy ; - optionally vacuum homogenization of the ingot by annealing at high temperature (above 900 ° C); - first quenching; - mechanical shaping (rolling, drawing, machining, or equivalent); - Thermal treatment of beta solution remelting beyond the temperature of the beta transus (up to a second temperature and then maintaining it for a certain duration); - second quenching; characterized in that said heat treatment phase is carried out under gaseous atmosphere, and also constitutes a surface treatment capable of forming on the surface, a layer of nitride, carbonitride, oxide, oxynitride, or equivalent.

Description

The present invention relates to a process for producing a titanium-based alloy having super-elastic and / or shape-memory properties, and intended to constitute devices used in the manufacture of a titanium-based alloy. the biomedical field, such as, for example, endodontic files, bows, wires and other orthodontic springs, dental implants, cardiovascular or pulmonary stents, guidewires and catheters for cardiovascular surgery, staples and joint prostheses for orthopedic surgery. The invention also relates to an alloy obtained by the method, and also to biomedical devices incorporating such an alloy.

In the biomedical field, devices and elements, such as those mentioned above by way of example, must have very different characteristics, on the one hand from the mechanical point of view given the objective of acting on a part of the human body (such as a tooth, for example in the context of an orthodontic application), and secondly on the biological level, to avoid, or minimize as much as possible, the reactions or consequences related to the contact of the device with the part of the human body or the organ.

Among the desired technical characteristics, even necessary, we can mention: a field of elasticity recoverable the widest possible (property of super elasticity), low stiffness, excellent chemical biocompatibility, high resistance to corrosion and sterilization products ease of machining and cold working, as well as an increase in hardness and resistance to surface wear. In known manner, attempts have been made to reconcile these constraints which are globally contradictory with each other. The superelastic and / or shape memory alloys currently used in the biomedical field are of the titanium-nickel type. Nevertheless, it is known that nickel is allergenic for the body and can cause inflammatory reactions, despite the interest it has for its mechanical properties, in particular super elasticity and / or shape memory. Ti-Ni alloys, on the other hand, have poor machinability and lead to premature rupture of the endodontic files (see, for example, Oiknine M., Benizri J., REV ODONT, STOMATO, 36 (2007) 109-123). sometimes difficult to get in cold form. These known alloys have a superelastic character by the stress destabilization of the parent (cubic) beta phase by transformation into a reversible alpha "(orthorhombic) martensitic phase (Kim HY, Ikehara Y., et al., ACTA MATERIALIA 54 (2006)). In addition, there are known nickel-free titanium-based alloys (referred to as "Gum Metals", Saito T., Furuta T. et al., SCIENCE 300 (2003) 464-467) which are considered being super elastic because, although they do not have martensitic transformation under stress, they have a low stiffness and a very considerable recoverable elasticity. Furthermore, it is known from French patent 2,848,810, by the US patent application 2007 / 0137742 and the patent application WO 2005/093109 alloys based on titanium and nickel-free.

Nevertheless, the alloys proposed in this prior art do not respond globally satisfactorily to all the required criteria, both mechanically and biocompatibility, especially at the surface. For example, in terms of biological compatibility, the above-mentioned French patent proposes a surface treatment of the alloy by nitride deposition, and this by a plasma-based technique. However, this known technique is not satisfactory. Indeed, the deposition using plasma does not allow a deposit of a uniform nitride layer. This has damaging or annoying consequences in the case of devices or elements having particular shapes or parts or areas difficult to access (such as concavities or the like).

In addition, said French patent describes a method that does not apply to a shape memory alloy and / or super elastic. While most alloys are based on titanium and nickel, it has recently been proposed superelastic alloys based on nickel-free titanium and particularly well cold-deformable. The article published in the journal JOURNAL OF THE MECHANICAL BEHAVIOR OF BIOMEDICAL MATERIALS 3 (2010) 559-564, by Bertrand E., Gloriant T. et al "Synthesis and characterization of a new superelastic Ti-25Ta-25Nb biomedical alloy" shows Thus, the process of the invention makes it possible to solve the problems of the prior art by proposing to produce a titanium-based alloy for biomedical applications with super-elastic properties. and / or shape memory, surface-treated, and which meets all the mechanical requirements, expressed above, and further which is improved with respect to the prior art, with regard to the superficial hardness, ease of machining and cold working, also resistance to sterilization, while being perfectly biocompatible.

To this end, according to the invention, the process for manufacturing a titanium alloy with super elastic properties and / or shape memory for the manufacture of a device for biomedical application, of the type comprising: - a vacuum melting preparation phase of the various constituent metals of the desired alloy, to achieve a mass such as in the form of an ingot; an optional vacuum homogenization phase of the ingot by annealing at a first temperature (greater than 900 ° C.) consisting of a rise in temperature of the ingot, and then maintaining at this temperature for a time allowing complete homogenization; - a phase of first tempering; a phase of mechanical shaping of the ingot into a final shape corresponding to the device to be manufactured (the forming being done by rolling, drawing, machining, or the like); a phase of heat treatment for redissolving in the beta phase beyond the beta transuscent consisting of a rise in temperature to a second desired temperature, of a maintenance at this temperature for a certain duration; a phase of second quenching; characterized in that said heat treatment phase is carried out under gaseous atmosphere, and also constitutes a surface treatment operation by reaction with said gas, so as to form homogeneously at the surface, a layer of nitride, carbonitride, oxide, oxynitride , or equivalent. The ingot obtained in the first step has a mass ranging from a few tens to a few hundred grams to realize the biomedical device.

Advantageously, said gas is nitrogen. Thus, the process of the invention introduces a surface treatment step (by gas phase nitriding) in order to improve the mechanical properties and surface biocompatibility of the alloy. During the nitriding step, care has been taken to install the shaped ingot in an enclosure and such that its arrangement allows the deposition of the layer in the gas phase over its entire surface, including in the concavities of the ingot shaped. To do this, the ingot is suspended by a chain in the heart of the enclosure constituting the oven. The first and second quenching aims to retain the beta phase at room temperature, to obtain the desired super elastic effect.

Depending on the composition of the alloy, a quenching with water or an air quenching is carried out. The homogenization phase is optional in that, for certain alloy compositions, the melting phase can lead directly to obtaining a homogeneous ingot. The homogenization phase is carried out at a temperature above 900 ° C. Furthermore, it is known that the "transus beta" temperature is the lowest temperature at which a 100% "beta" phase of the alloy can exist. It varies between 600 ° C and 1050 ° C, depending on the composition of the alloy. Preferably, the simultaneous phase of redissolution in beta phase and nitriding (in the case where the gas is nitrogen) is carried out at a temperature of between 600 and 1050 ° C., preferably between 800 ° C. and 1050 ° C. ° C, under a gaseous atmosphere, preferably nitrogen, for a period of a few hours, to obtain a nitride surface layer ranging from a few microns to tens of microns thick, depending on the intended use. The duration of maintenance at the annealing temperature of the homogenization phase is between 12 hours and 20 hours, and preferably of the order of 16 hours.

Alternatively, the nitride layer (or any other type of layer) can be made by a technique of the type: - plasma - ion implantation - cathode arc - laser - any PVD or CVD process. The technique employed must not modify the metastable beta microstructure of the alloy which is at the origin of the super elastic effect.

Advantageously, the nitriding phase is conjugated with a simultaneous recrystallization phase and leads to obtaining a recrystallized beta microstructure.

The invention also relates on the one hand to an alloy as obtained by the process above, and on the other hand to a device for biomedical use, incorporating said alloy.

According to a preferred form, the alloy of the invention comprises, according to its chemical composition in atomic percentage: - Titanium: between 30% and 98% - Niobium: between 0% and 40% - Molybdenum: between 0% and 15% - Chromium : between 0% and 15% - Iron: between 0% and 15% - Zirconium: between 0% and 40% - Hafnium: between 0% and 40% - Tantalum: between 0% and 60% - Oxygen: between 0% and 2% - Nitrogen: between 0% and 2% - Silicon: between 0% and 2% - Boron; between 0% and 2% - Carbon: between 0% and 2% - Vanadium: between 0% and 15% - Tungsten: between 0% and 20% - Aluminum between 0% and 10% - Tin between 0% and 10% - Gallium: between 0% and 10% The following are mentioned some compositions of the alloy of the invention, given in atomic percentage: Ti74 Nb26 Ti72 Nb18 Taio Ti74 Nb20 Zr6 Ti76 Nb23 N Ti78.5 Nb15 Zr2.5 Sn4 Thus, unlike the prior art, the alloy of the invention does not contain nickel. The invention will be better understood in the light of the following description of illustrative but nonlimiting examples on the basis of the appended drawings in which: FIG. 1 is a graphical representation of the variation of the temperature as a function of time at which is subjected to the alloy ingot, according to the different steps of the method of the invention. FIG. 2 is an optical microscope micrograph showing the metastable beta microstructure of an exemplary Ti-25Ta-25Nb alloy core. FIG. 3 shows a cross section, observed by optical microscopy, of the microstructure of the nitrided surface of the alloy of FIG. 2. FIG. 4 shows a cycled uniaxial traction curve (successive charge / discharge) showing the superelasticity of the nitrided alloy by the hysteresis formation between charge and discharge due to stress-induced martensitic transformation - Figures 5A and 5B show respective optical micrographs of a non-nitrided sample (prior art) and nitrided (according to the invention) after a pion-disk tribometer scratch test at load and with equivalent number of cycles The invention is now described with reference to FIG. 1 which shows a schematic curve of the variations of temperature of an ingot including different compounds, and intended to constitute the alloy, as a function of time, with regard to the successive steps of the process of the invention n) In a preliminary step (not shown in FIG. 1), the various quantities of metals which will be included in the composition of the alloy to be produced are collected in proportions defined below. The metal mixture is subjected to a preliminary melting operation at a temperature of between 2000 ° C and 3000 ° C. Advantageously, this preliminary melting step is carried out in a cold crucible by semi magnetic levitation using a high frequency induction generator. It is also possible to use sintering techniques, conventional or fast (flash). During this preliminary step, it is important to ensure a homogeneous mixture, without any inclusion or contamination or pollution of foreign elements. In this respect, the melting is preferably carried out under vacuum or in a controlled atmosphere with a neutral gas (such as argon for example). The various elements likely to enter the composition of the ingot and thus the future alloy include: - titanium, for a large part or a majority part - other metals: tantalum, niobium, molybdenum, zirconium, hafnium, vanadium, iron , chromium, tungsten which constitute elements called "beta-stabilizers", and possibly - still other elements such as: aluminum, silicon, boron, carbon, oxygen, nitrogen, tin, gallium ... which, added in small quantities , are likely to improve the properties of super elasticity.

The various components are chosen in qualitative and quantitative terms in order to achieve a metastable beta-type alloy by quenching, and allowing the formation of a martensitic phase alpha ", which is reversible, and thus providing super elastic properties and / or shape memory.

The "beta-metastable" character of the alloy results in a low apparent modulus of elasticity ranging from 10 GPa to 70 GPa approaching that of the bone.

From the fused ingot, obtained by the preliminary melting step mentioned above, the second step consists of a "homogenization annealing" carried out at high temperature (typically between 900 ° C. and 1200 ° C.) under ultra-high vacuum.

Note that on the graph of Figure 1, the time scale (abscissa) is not respected, for practical reasons. The ingot is maintained at the so-called annealing temperature for a period that may be several hours. The temperature and the duration of the treatment depend on the alloy considered. In the end, a completely homogeneous alloy must be obtained. The homogenization annealing phase is completed by a first quenching, preferably by quenching with water, so as to retain a metastable "beta" microstructure at room temperature which will favor the shaping operations. The quenching is carried out in a fraction of a second by dropping the ingot from the annealing temperature into a tray of water at room temperature.

Then, we perform the operations of shaping and machining, cold, that is to say at room temperature. This step aims at conforming the ingot according to the shape of the desired device or element, for a biomedical application as mentioned above.

The shaping operation is of course adapted to the configuration and shape of the product to be produced and uses known machining techniques, or shaping such as drawing, rolling, extrusion or other.

The biomedical device thus manufactured, consisting of the desired alloy, is then subjected to a heat-treatment solution in solution above the temperature T p which is the transus temperature typically between 600 ° C and 1050 ° C.

The device is maintained at this temperature for a duration of a few tens of minutes to a few hours, and this, under gaseous atmosphere such as nitrogen for example. The apparatus used is an oven known in itself.

This treatment conducted at constant temperature, has two objectives: 2 9826 18 12 - obtaining a "recrystallized beta" microstructure having a refined grain size in order to improve and optimize the mechanical properties of the final biomedical device; the deposit, during its stay in the oven, of a nitride layer on the surface of the device, by the direct hot reaction, between the alloy constituting the device, and the nitrogen gas introduced into the furnace; tempering. This treatment is a nitriding process in the gas phase. The duration of this nitriding step varies between 0.5 and 10 hours depending on the alloy compositions, desired thicknesses and the shape of the device. The temperature maintained during this nitriding step is between 600 and 1050 ° C. Finally, at the end of the nitriding / recrystallization phase, a second quenching, preferably quenching with water, is carried out, which brings the temperature of the device to room temperature. This second quenching makes it possible to maintain the beta microstructure of the alloy in a metastable form. The Applicant has performed laboratory tests on alloy samples made according to the method of the invention described above. The alloy in question is super-elastic of the recrystallized beta-metastable type, with a grain size of between 10 and 60 microns (see the beta microstructure of the core of the alloy in FIG. 2). Its composition, expressed as mass percentage is: Ti (50%), Ta (25%) and Nb (25%). The recrystallization-nitriding phase was carried out at 800 ° C. for a period of 3 hours and led to a titanium nitride layer a few tens of microns thick. The microstructure of the surface nitride is shown in cross-section in Figure 3, where the dark portions correspond to the nitride zone consisting of nitrogen-rich needles (internal nitriding).

The thin nitride layer does not alter the super elastic properties of the alloy thus obtained. Figure 4 shows the cyclic tensile curve for the nitrided alloy which shows the presence of charge / discharge hysteresis characteristic of the superelastic effect. The cycled tensile test presented in this figure was conducted on a flat test-tube one millimeter thick obtained by the present method of manufacture. The charge / discharge cycles were carried out with an incrementation of 0.5% in deformation.

A nitrided alloy produced by the process of the invention has, compared with an alloy of the same composition, non-nitrided, an increase in the surface hardness (measured 4 times higher by micro hardness Vickers), which causes a very strong increase in the wear resistance (used volume reduced by 85%) as well as a clear reduction in the coefficient of friction (divided by 5). Figures 5A and 5B each show a top view of a sample of alloy having undergone a scratch test respectively nitride (5A - prior art) and nitride (5B according to the invention). The furrows (dark in color) are obtained under load of 25g after 200 cycles in rotation using a pin-to-disk tribometer. It is found that the alloy according to the invention (FIG. 5B) has a much higher surface resistance. The nitriding phase performed in the gas phase as described above has many advantages over known nitriding techniques. The deposition of a nitride layer of substantially homogeneous thickness, even on objects of complex shape, as are in most cases also devices for biomedical use; 30 - Great simplicity of implementation; Nitriding (that is to say the deposition of a nitride) is carried out simultaneously to the recrystallization of the alloy during the dissolution; this is not possible by the other nitriding methods which supposes to carry out these only after recrystallization under vacuum. - A very good adhesion of the layer on the alloy by formation of internal nitriding near the surface. In addition, the second quenching of the process of the invention, which follows the simultaneous nitriding / recrystallization step, has the advantage of maintaining the metastable beta microstructure in the core of the alloy, to obtain the super elastic effect. The invention is not limited to the deposition of nitride, but also includes the deposition of surface layer of oxide, oxynitride, or carbonitride.

In this case, a suitable gas or gas mixture is used, in this case oxygen for an oxide layer, or by adding carbon dioxide, nitrogen monoxide or even air to obtaining oxynitride or carbonitride on the surface.

The alloy made according to the process of the invention, and according to the compositions indicated, has advantages according to the alloys of the titanium / nickel type and in particular a very high capacity to cold deformation, and superior machinability these two advantages being particularly appreciated in the case of devices with biomedical application. For example, endodontic files have in the prior art relatively high wear and significant risk of fractures caused by machining streaks. In addition, the cutting capacity of the alloy according to the invention is greater than that of the alloys of the prior art. The nitride layer enhances the hardness and wear resistance properties while providing biocompatibility benefits. In the cardiovascular application devices, such as stents, there is observed, as regards the alloy of the invention, a better haemocompatibility. Finally, the biomedical device thus produced is more resistant to sterilization operations and is less sensitive to bacteriological activity thanks to the presence of the nitriding layer. In summary, the process according to the invention makes it possible to manufacture a titanium-based alloy for biomedical application, which has super elastic and / or shape memory properties, with all the advantages that derive from it and expressed above, and in addition, these properties are for some of them reinforced by the nitride layer, which itself provides other capabilities or mechanical properties, and finally the latter enhances or improves the biocompatibility of the device for biomedical use.

Claims (11)

REVENDICATIONS1. A method of manufacturing a titanium alloy with super elastic properties and / or shape memory for biomedical application, of the type comprising the phases of: - preparation by vacuum melting of the various constituent metals of the desired alloy, for make an ingot; - optionally vacuum homogenization of the ingot by annealing at a first temperature, especially greater than 900 ° C, consisting of a temperature rise of the ingot, a maintenance at this temperature for a period for complete homogenization - first quenching; mechanical shaping such as by rolling, wire drawing, machining, or the like; - Thermal treatment of beta solution in solution beyond the transus beta consisting of a rise in temperature to a desired second temperature, a maintenance at this temperature for a certain duration - second quenching; characterized in that said heat treatment phase is carried out under a gaseous atmosphere, and also constitutes a so-called nitriding phase, performing a surface treatment by reaction with said gas, so as to form a nitride layer uniformly at the surface, carbonitride, oxide, oxynitride, or the like. 2. Method according to claim 1, characterized in that the nitriding phase is carried out at the second temperature between 600 and 1050 ° C, preferably between 800 ° C and 1050 ° C.30. 3. Method according to claim 1 or 2, characterized in that the nitriding phase is carried out under a nitrogen atmosphere. 4. Method according to one of claims 1 to 3, characterized in that the duration of maintenance at the annealing temperature of the homogenization phase is between 12 hours and 20 hours, and preferably of the order of 16 hours. 5. Method according to one of claims 1 to 4, characterized in that the gas phase nitriding phase is carried out by plasma, ion implantation, cathode arc, laser, or PVD or CVD process. 6. Method according to one of claims 1 to 5, characterized in that the nitriding phase is conjugated with a simultaneous recrystallization phase. 7. Method according to one of claims 1 to 6, characterized in that the homogenization phase is carried out at a temperature above 900 ° C. 8. Method according to one of claims 1 to 7, characterized in that the first and second quenching are performed in the form of quenching with water or air. 9. Alloy made by the process according to one of the preceding claims. 10. Alloy according to the preceding claim, characterized in that it comprises according to its chemical composition in atomic percentage: - Titanium: between 30% and 98% - Niobium: between 0% and 40% - Molybdenum: between 0% and 15% - Chromium: between 0% and 15% - Iron: between 0% and 15% - Zirconium: between 0% and 40% - Hafnium: between 0% and 40% - Tantalum: between 0% and 60% - Oxygen: between 0 % and 2% - Nitrogen: between 0% and 2% - Silicon: between 0% and 2% - Boron; between 0% and 2% - Carbon: between 0% and 2% - Vanadium: between 0% and 15% - Tungsten: between 0% and 20% - Aluminum between 0% and 10% - Tin between 0% and 10% - Gallium: between 0% and 10% 11. Device for biomedical use, incorporating or consisting of the alloy according to claim 9 or 10.